The exchange of email over the Internet and other networks has become an ordinary activity in world of business and also in people's personal lives. Unfortunately, the lack of a real cost of sending an email to a receiver's electronic address, combined with the increasing availability of such address lists, leads to a high percentage of undesirable emails. For the receiver and the intermediary service providers, the undesirable emails, which have become known as spam, result in a significant expenditure of time and money in order to extract the desirable messages from the background noise of undesirable ones. For the senders, the same phenomenon decreases the value of email as a communication channel, as any desirable messages they send to receivers tend to become lost in the sea of spam or otherwise ignored by the receivers.
One way to fight spam is to use technical means to filter out the undesirable emails from all of the received emails. With some known email filtering programs, the header of the sent email, its title or subject, and/or its content is analyzed.